The present invention relates to a method and to a device for displacing a moving body on a base mounted elastically with respect to the ground.
In the context of the present invention, said device comprises, in addition to the moving body and the base, a controllable actuator, for example an electric motor, intended to bring about a linear displacement of the moving body on the base.
The latter is mounted on the ground via elastic mounts which are intended to isolate it from vibration originating from said ground.
However, when the moving body is made to move, during its acceleration and deceleration phases, the base is subjected to the reaction of the force applied to the moving body by the actuator. This reaction force excites the base which then oscillates on its supports. This disturbs the relative position of the moving body with respect to the base, and has a very harmful effect on the precision of the device.
This error in relative position remains after the end of displacement of the moving body and does not disappear until after the base is stabilized (which takes place a great deal later).
Various solutions for overcoming this drawback are known. Some of these solutions in particular envisage:
immobilizing the base during the acceleration and deceleration phases using a locking system, for example an electromagnetic locking system, which is mounted in parallel with the elastic mounts. This known solution does, however, prevent the mounts from isolating the base from vibration originating from the ground during said acceleration and deceleration phases;
canceling the effect produced by the force generated by the actuator by providing an additional actuator which is arranged between the base and the ground and which generates an additional force of the same amplitude but of opposite sense; or
displacing an additional moving body on the base with a similar displacement but of opposite sense with respect to the displacement of the moving body, so as to cancel the inertial effects.
However, none of these known solutions is satisfactory because their effectiveness is low and they all require additional means (locking system, additional actuator, additional moving body) which, in particular, increase the complexity, cost and bulk of the device.
The object of the present invention is to overcome these drawbacks. The invention relates to a method for displacing a moving body extremely precisely and at a low cost on a base mounted elastically with respect to the ground, said moving body being displaced linearly with a predetermined displacement in terms of distance and time, under the action of a controllable force.
To this end, said method is noteworthy according to the invention in that:
a) a mathematical relationship illustrating the movements of the entity formed by the base and the moving body and dependent on parameters related to the base and to the moving body and which vary during a displacement of the latter, is defined;
b) using said mathematical relationship, mathematical processing is used to determine a force which, applied to said moving body, produces a combined effect:
on the one hand, on the moving body so that it performs said predetermined displacement; and
on the other hand, on the base at least so that it is immobile at the end of the displacement of the moving body; and
c) the force thus determined is applied to said moving body.
Thus, the force applied to the moving body allows the latter to effect the envisaged predetermined displacement, particularly in terms of time and in terms of distance, while at the same time rendering the base immobile at the end of the displacement so that it does not oscillate and does not disrupt the relative positioning between it and the moving body.
In consequence, because of this combined effect or control of the moving body and of the base, an extremely precise displacement of the moving body is obtained in a frame of reference that is independent of the base and linked, for example, with the ground.
It will be noted that the implementation of the method according to the invention is not restricted to a displacement along a single axis, but can also be applied to displacements along several axes which may be considered as being independent.
In the context of the present invention, it is possible to use various mathematical relationships, particularly a mathematical relationship obtained, for example, by deconvolution.
However, in a preferred and simplified embodiment, by way of mathematical relationship in the aforementioned step a), use is advantageously made of the following relationship:   y  =      x    -                  k1        k0            ⁢              xe2x80x83            ⁢              x        .              +                  1        m            ⁢              xe2x80x83            ⁢              (                  mB          -                                    k1              2                        k0                          )            ⁢              xe2x80x83            ⁢      xB        -                            mB          m                ·                  k1          k0                    ⁢              xe2x80x83            ⁢              x        .            ⁢              B        .            
in which:
y represents an imaginary variable;
k0 and k1 respectively represent the natural frequency and the damping of the elastically mounted base;
m and mB represent the masses of the moving body and of the base, respectively;
x and xB represent the displacements of the moving body and of the base, respectively; and
{dot over (x)} and {dot over (x)}B represent the speeds of displacement of the moving body and of the base, respectively.
As far as step b) of the method according to the invention is concerned, various mathematical processing operations may be performed, particularly processing operations which employ:
a sum of sinusoidal functions; or
a polynomial decomposition.
For this last type of processing operation, the force F is advantageously determined from the following relationship:   F  =            m      ⁢              xe2x80x83            ⁢              y        ¨              +          m      ⁢              k1        k0            ⁢              xe2x80x83            ⁢              y                  (          3          )                      +                  mmB        k0            ⁢              xe2x80x83            ⁢              y                  (          4          )                    
ÿ, y(3) and y(4) being the 2nd, 3rd and 4th-order derivatives of the variable y, respectively, and using the following polynomial decomposition of y(t):
y(t)=y0+(y1xe2x88x92y0)s5(126xe2x88x92420s+540s2xe2x88x92315s3+70s4)
with       s    =                  t        -        t0                    t1        -        t0              ,      y0    =                  y        ⁢                  xe2x80x83                ⁢                  (          t0          )                ⁢                  xe2x80x83                ⁢        and        ⁢                  xe2x80x83                ⁢        y1            =              y        ⁢                  xe2x80x83                ⁢                  (          t1          )                      ,
t being time and t0 and t1 being the instants of the start and end of the displacement of the moving body.
The present invention also relates to a device of the type comprising:
a base mounted elastically with respect to the ground;
a moving body which can be displaced linearly on said base; and
a controllable actuator capable of applying a force to said moving body with a view to displacing it on said base.
According to the invention, said device is noteworthy in that it additionally comprises means, for example a computer:
which carry out steps a) and b) of the aforementioned method to calculate a force which, applied to said moving body, makes it possible to obtain the aforementioned combined effect; and
which determine a control command and transmit it to said actuator so that it applies the force thus calculated to the moving body at the time of displacement.
Thus, in addition to the aforementioned advantages, the device according to the invention requires no additional mechanical means, and this reduces its cost and bulk and simplifies its production, compared with the known and aforementioned devices.